Playing with fire ... Previews of the National Theatre's Greenland met with harsh reviews online. Photograph: Tristram Kenton for the Guardian

A few days ago on this blog, Alistair Smith argued for the redundancy of theatre previews on the basis that they benefit neither audiences, who pay close to full-whack, nor producers, who leave themselves at the mercy of the blogosphere. It's an appealing argument, isn't it? Greed and the internet scupper tradition.

Appealing, but flawed. What Smith has done is to dismiss a system on the basis of its misuse. It's like calling for the abolishment of pedestrian crossings because some people skip red lights. In the case of previews, it's the work itself – and, by extension, audiences – that benefit. Only by putting work in front of an audience can theatre-makers understand what has been made and how it functions. Without gauging audience response, they have only worked in theory. They're reliant on the assumption that others will see the work as they do in the rehearsal room, in other words, that everything will be received as imagined. Previews allow practitioners – including directors, actors, designers and production managers – to check their workings and ensure their intentions are actually being achieved as planned.

Theatre is more than a matter of individual moments reading correctly, and previews also serve a more intangible function: that of timing and timbre. These are the nuances of a show that drastically affect the experience of those watching. They cannot be fine-tuned until the whole is in place. Moreover, we're forever harping on about the effect that an audience has on live work. Dropping the preview system is akin to sitting an exam without ever having practised under exam conditions. The practice paper itself can only help so much.

Rather than calling for its abolition, then, we should be demanding respect for the preview system from both producers and audiences. The former must admit that the product is unfinished and do its test-audiences the courtesy of a discount. Audiences can vote with their wallets; if the discount offered (or not) is not recompense enough, then wait until after opening night. No one is forced to attend a preview. Equally, bloggers must stop the cynical practice of reviewing previews. They do so, first, to save money – an argument undermined given that the practice is often justified by the lack of discount – and second, to chase hits by beating the mainstream media.

Fair enough, you may think; bloggers aren't subject to the same professional etiquette as critics. They do, however, have ethical responsibilities. To write about any performance is to concretise it. It is to hold up an instance as representative of all others. Identifying that instance as a preview in no way absolves the writer. To admit that changes or improvements might occur does not prevent that process of concretisation, since reviews cannot change as the production does. Admission of potential alterations does not accommodate actual ones. The preview still stands for the show. Ultimately, to review a preview (where that term is not exploited, as it has been in Spider-Man: the Media Circus) is to show a lack of respect for those involved. Criticism must start from a foundation of respect. If bloggers want the same regard as critics, they must approach their subjects likewise. At the very least, that involves upholding the integrity of the preview system.